DESCRIPTION Major trauma can cause volumetric muscle loss (VML) resulting in life-long disability. Although skeletal muscle is capable of remarkable regenerative potential, when injury is massive and destroys the underlying architecture, regeneration is aborted and is characterized instead by scar tissue formation. The standard of care in such injuries is wound closure, leaving little hope for functional recovery. The promise of regenerative medicine is the full regeneration of damaged tissues, either by promoting repair from endogenous stem cells or by the transplantation of cells to enhance regeneration. Perhaps the best example of this is skin grafting in the setting of massive tissue loss in burn victims. The fact that grafted skin contains endogenous stem cells assures that the graft will not only restore function acutely but also chronically as the stem cells function to replenish skin cells that are lost during the normal turnover of the tissue. Likewise, the long-term goal of regenerative medicine is to be able to restore damaged tissue and maintain that tissue for the full lifetime of the individual. Major advances have been made in the culture and transplantation of muscle stem cells (MuSCs, also known as satellite cells) in recent decades, primarily in rodent models of muscle injury and degenerative disease. It has been known for over 40 years that transplanted myoblasts, the more differentiated progeny of MuSCs, can contribute to new muscle formation in the host. However, it has long been recognized that those cells have limited regenerative capacity. There have very few studies, however, of the importance of physical activity following MuSC transplantation, and previous studies suggest that physical activity can profoundly influence the host environment, rendering that environment much more hospitable to stem cell engraftment. The goals of the studies outlined in this proposal are to evaluate the effects of physical activity in the form of voluntary wheel running, on the efficacy of MuSC transplantation therapy for VML. In this proposal, we will use standardized injuries to host mouse muscle producing VML, and we will test for regenerative capacity of transplanted cells, with or without physical activity followng transplantation, to restore tissue structure and function. We will also test the importance of the biological scaffold used to seed the MuSCs for transplantation given recent results that biological scaffolds alone may promote muscle restoration in animal models of VML. Analyses will include non-invasive imaging, physiological muscle function testing, and both histological and immunohistochemical analysis of tissue regeneration. The overall goal of this proposal is to establish whether physical activity can enhance MuSC transplantation for the treatment of volumetric muscle loss, and to explore the importance of the timing of onset and duration of that activity. This will have direct and immediate relevance to Veterans who are suffering from skeletal muscle injuries, injuries that have limited their functional capacity and that, to date, hve no hope of further recovery. Our goal is to develop a therapeutic approach to muscle tissue repair based upon a deep understanding of the basic stem cell biology, combined with experimental evidence of rehabilitation therapy, to improve the health and quality of life of Veterans whose function and further rehabilitation is limited by the lack of effective therapeutic options.